Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in what is called a projection exposure apparatus, which comprises an illumination device and a projection lens. The image of a mask (reticle) illuminated by the illumination device is in this case projected by the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In projection lenses designed for the EUV range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process. Typical projection lenses designed for EUV, as known e.g. from U.S. Pat. No. 7,538,856 B2, may have for example an image-side numerical aperture (NA) in the range of NA=0.2 to 0.3 and image an (e.g. ring-segment-shaped) object field into the image plane or wafer plane.
Here, inter alia, the operation of mirrors under grazing incidence is also known. Such mirrors operated under grazing incidence, which it is desirable to use chiefly in respect of the comparatively high obtainable reflectivities (e.g. of 80% and more), are understood to mean e.g. mirrors for which the reflection angles, which occur during the reflection of the EUV radiation and relate to the respective surface normal, are at least 65°. Such mirrors are also referred to as GI mirrors (“grazing incidence”).
The increase of the image-side numerical aperture (NA) and the realization of arrangements using GI mirrors are typically accompanied by an enlargement of the required mirror areas of the mirrors used in the projection exposure apparatus.
In particular, the use of computer-generated holograms (CGH) is known for highly precise testing of the mirrors.
FIG. 5 initially shows a schematic illustration for explaining a possible functional principle of a conventional interferometric test arrangement for testing a mirror 501.
In accordance with FIG. 5, an interferogram between a reference light (reference wave) that is reflected at a reference surface 510 (“Fizeau plate”) and a measurement light (test wave) that is reflected at the mirror 501 is produced in a Fizeau arrangement. Here, the measurement light is formed into an aspherical wavefront by a computer-generated hologram (CGH) 520, said wavefront corresponding mathematically exactly to the “test object form” (i.e. the form of the relevant mirror 501) at an intended distance. The wavefronts reflected firstly by the reference surface 510 and secondly by the relevant mirror 501 or test object interfere with one another in an interferometer 505 (which is schematically illustrated by way of example in terms of its overall design in FIG. 6), with a collimator 509, a beam splitter plate 508, a stop 507, an eyepiece 506 and a CCD camera 504 and a light source 503 for the interferometer 505 being depicted in FIG. 6. An interferogram of the respective mirror is recorded by the CCD camera 504.
The problem occurring here in practice with increasing mirror dimension, in particular in the case of GI mirrors or concave mirrors, is that the realization of ever larger CGHs has limits, with typical CGH dimensions being able to be e.g. 6 inch (=15.24 cm) or 9 inch (=22.86 cm). Although, in this respect, it is possible to use a plurality of CGHs for different, successively set mirror positions to reduce the required CGH dimensions, additional practical problems emerge in this case, with, in particular, the lengthening of the time duration required for the entire test and also the necessity of an exact combination of the measurement results obtained for the different mirror regions being mentioned.
Moreover, in the case of using a plurality of CGHs for testing one and the same mirror, it was found to be increasingly difficult to reliably distinguish between the manufacturing errors that are typically present in the CGHs and the mirror errors that are to be ascertained within the scope of the test, as a result of which the accuracy of the test is impaired. If a plurality of CGHs are used, further difficulties emerge from the relative degrees of freedom in terms of adjustment (i.e. distances and relative orientation) between CGH and mirror.
Moreover, the realization of a calibration of the CGHs that are used in the mirror test by using so-called complex-encoded CGHs is known, wherein at least one further “calibration functionality” for providing a reference wavefront that serves for calibration or error correction is encoded at the same position in one and the same CGH in addition to the “use functionality” (i.e. the CGH structure that is designed in accordance with the mirror form for forming the wavefront that mathematically corresponds to the test object form) that is required for the actual test.
In respect of the prior art, reference is made in a purely exemplary manner to U.S. Pat. No. 7,936,521 B2, U.S. Pat. No. 8,089,634 B2 and the article Beyerlein, M.; Lindlein, N.; Schwider, J.: “Dual-wave-front computer-generated holograms for quasi-absolute testing of aspherics”, Appl. Opt. (USA) 41, page 2440 (2002).